The establishment of a radioimmunoassay which will allow the detection and quantitation of assembled attack complexes on cell surfaces or in fluid phase will be an important research goal. A double antibody system will be explored in which a known amount of complex specific antibody will be incubated with 125I-radiolabeled C5b-9 and a serum or cell sample. All tests will be performed in the presence of WHS. If the incubation contains cells, they will be removed by centrifugation. In a second stage reaction, goat-anti-rabbit Ig will be added to the mixtures, and after an overnight incubation at 4 degrees C, the precipitates will be collected and 125I radioactivity determined. The presence of C5b-9 in the original sample will be reflected by a decrease in 125I radioactivity recovered in the final step and can be quantitated by comparison to a standard inhibition curve. Characterization of the newly described C5b-9 receptor on lymphoid cells will also be conducted. What lymphocyte sub-populations possess these receptors? How many receptors are present per cell? What relationship do these receptors have to the C3b, C3d or C4b lymphocyte receptors? Do these receptors react with partially assembled attack complexes or do they react only with fully assembled C5b-9 complexes? Can preformed C5b-9 in the fluid phase bind to lymphocytes or must the complex be bound to a membrane? What metabolic consequences occur to lymphocyte metabolism upon binding of membrane bound C5b-9 on their surface? DNA synthesis? Lymphokine production? These are all questions to be examined during the coming year. BIBLIOGRAPHIC REFERENCE: Kolb, W. P., and H. J. Muller-Eberhard. The membrane attack mechanism of complement: The three polypeptide chain structure of the eighth component (C8). J. Exp. Med., 1976, in press.